1. Field of the Invention
The invention relates to tension bands and methods and apparatus for manufacturing such tension bands. The invention is concerned in particular with tension bands for use as supports in cryogenic applications.
2. Description of the Related Art
Superconducting magnets are commonly used in magnetic resonance imaging (MRI) systems and are essentially solenoids which are cooled to operational temperatures in a bath of liquid helium provided in a cryostat. Liquid helium boils at a temperature of 4.2 K. and to maintain this helium bath it is necessary to design a cryostat that reduces to a minimum the transfer of heat from the environment. Three methods of heat transfer are usually considered: conduction plus convection through gas (air) between the helium vessel and its environment, radiation from the environment onto the helium vessel, and conduction through the physical support structure of the helium vessel.
To deal with the problems of heat transfer, cryostats built to contain imaging magnets are built with a series of radiation shields between the surface of the outer vessel and the helium vessel. In some configurations of imaging cryostats the intermediate shields are cooled by refrigerators that use external power. Others use as an intermediate shield a vessel which contains liquid nitrogen which boils at 77 K. at normal atmospheric pressure. In this case, the shield is maintained at an intermediate temperature by the boiling nitrogen and energy from the environment is absorbed by the latent heat of boiling.
It is necessary to design a support system for the elements of the cryostat which is strong enough to support the vessels and shields and which does not allow heat to be conducted directly from the outer vessel to the helium can. The forces which the elements of the cryostat undergo include gravity, acceleration during movement of the cryostat, and magnetic interactions with the structural iron.
Conventionally, a system of struts operating in three dimensions and supporting the vessels while acting in tension has been used for NMR imaging magnet cryostats. Examples of these struts are described in USSN 912,246. Unique combinations of strength and thermal isolation are required for NMR magnets.
The weight of the imaging magnet and vessel range from 1500 to 4000 kilograms dependent upon magnet field strength and vessel configuration.
Until recently, support systems were designed solely to deal with gravitational forces. Relatively recently, it has become an operational requirement that imaging magnets are built into mobile scanners housed in custom built bodies on air ride trucks. The vibration and impact accelerations experienced by the magnet can occur in any of three dimensions.
These requirements have resulted in the need for a support member capable of withstanding a sustained fatigue loading of 15 to 30 KN, with an ultimate tensile strength of greater than 75 KN. This represents 2 g fatigue and 5 g shock loading, for the heaviest magnet considered, when distributed across 4 suspension members.
A paper entitled "Fatigue Resistance of a Uniaxial S-Glass/Epoxy Composite at Room and Liquid Helium Temperatures" by Tobler and Read in J. Composite Materials, Vol. 10 (January 1976), p.32 and a paper entitled "Filament Wound Composite Thermal Isolator Structures for Cryogenic Dewars and Instruments" by Morris in Composites for Extreme Environments, ASTM SDP768 N.R.A. Adsit, Ed, American Society for Testing Materials 1982, p. 95-109 describe composite materials for manufacturing tension bands which have been proposed in the past for satisfying the above requirements. These papers describe a variety of uses for such tension bands including their use as thermal isolator straps for cyogenic dewars and instruments.
Although these tension bands have a good thermal/tensile strength, they have to be produced individually at very high cost if they are to have sufficient strength for use in heavy duty applications such as cryostats for NMR imaging magnets.